The (X,Y) coordinates of the targets to be observed with multi-object
spectroscopy, can be obtained in two ways:

From the astrometry of the field. Transformation of the equatorial
coordinates of the targets onto the corresponding aperture plate
coordinates (X,Y), is performed with the LAPLATE programme, which is
described in Appendix F and which is a
modification of the APLATE programme of the STARLINK package, for the
geographical coordinates of the ORM. LAPLATE takes into account the
following parameters: telescope focal plane scale, thermal dilation
coefficient of the aperture plate material, astronomical coordinates for the
centre of the field, and possible proper motions of the targets. These are all
considered by the programme in order to obtain the appropriate
corrections of the X,Y coordinates.

By taking photographic plates of the field in advance. The positions
can be measured with a microdensitometer or other coordinate measuring
machine.

Several error sources can contribute to the field acquisition, for
example poor astrometry (the drilled holes for the stars do not coincide
with the apparent positions) or bad centering of the field and rotator
adjustment. Taking direct plates eliminates some sources of astrometric
error, at the expense of requiring extra observing time.

Besides the coordinates of the targets, at least three other field stars
must be chosen for telescope guiding. These stars must fulfil the following
requirements:

They must have magnitude 12 to 13 in V. Brighter stars are close to the Earth
and can have large proper motions, so that it is in general difficult to obtain
good astrometry of them. Less bright stars are difficult to observe.

They must have similar brightness. If not, simultaneous observation on
the monitor could be prevented as the brightest ones can saturate the
camera while the weakest remain unseen.

They must be as separated from each other as possible in the field, and,
preferably along the East-West direction which is free of atmospheric
refraction. This helps in obtaining a more precise field acquisition.

Finally, some positions free of stars must also be chosen, in order to
perform sky corrections.